Shashi Thutupalli

Chimera states in mechanical oscillator networks

The synchronization of coupled oscillators is a fascinating manifestation of self-organization that nature uses to orchestrate essential processes of life, such as the beating of the heart. Although it was long thought that synchrony and disorder were mutually exclusive steady states for a network of identical oscillators, numerous theoretical studies in recent years have revealed the intriguing possibility of “chimera states,” in which the symmetry of the oscillator population is broken into a synchronous part and an asynchronous part. However, a striking lack of empirical evidence raises the question of whether chimeras are indeed characteristic of natural systems. This calls for a palpable realization of chimera states without any fine-tuning, from which physical mechanisms underlying their emergence can be uncovered. Here, we devise a simple experiment with mechanical oscillators coupled in a hierarchical network to show that chimeras emerge naturally from a competition between two antagonistic synchronization patterns. We identify a wide spectrum of complex states, encompassing and extending the set of previously described chimeras. Our mathematical model shows that the self-organization observed in our experiments is controlled by elementary dynamical equations from mechanics that are ubiquitous in many natural and technological systems. The symmetry-breaking mechanism revealed by our experiments may thus be prevalent in systems exhibiting collective behavior, such as power grids, optomechanical crystals, or cells communicating via quorum sensing in microbial populations.

Swarming behavior of simple model squirmers

We have studied experimentally the collective behavior of self-propelling liquid droplets, which closely mimic the locomotion of some protozoal organisms, the so-called squirmers. For the sake of simplicity, we concentrate on quasi-two-dimensional (2D) settings, although our swimmers provide a fully 3D propulsion scheme. At an areal density of 0.46, we find strong polar correlation of the locomotion velocities of neighboring droplets, which decays over less than one droplet diameter. When the areal density is increased to 0.78, distinct peaks show up in the angular correlation function, which point to the formation of ordered rafts. This shows that pronounced textures, beyond what has been seen in simulations so far, may show up in crowds of simple model squirmers, despite the simplicity of their (purely physical) mutual interaction.

Synaptotagmin-1 may be a distance regulator acting upstream of SNARE nucleation

Synaptotagmin-1 triggers Ca2+-sensitive, rapid neurotransmitter release by promoting interactions between SNARE proteins on synaptic vesicles and the plasma membrane. How synaptotagmin-1 promotes this interaction is unclear, and the massive increase in membrane fusion efficiency of Ca2+-bound synaptotagmin-1 has not been reproduced in vitro. However, previous experiments have been performed at relatively high salt concentrations, screening potentially important electrostatic interactions. Using functional reconstitution in liposomes, we show here that at low ionic strength SNARE-mediated membrane fusion becomes strictly dependent on both Ca2+ and synaptotagmin-1. Under these conditions, synaptotagmin-1 functions as a distance regulator that tethers the liposomes too far from the plasma membrane for SNARE nucleation in the absence of Ca2+, but while bringing the liposomes close enough for membrane fusion in the presence of Ca2+. These results may explain how the relatively weak electrostatic interactions between synaptotagmin-1 and membranes substantially accelerate fusion.

Bilayer membranes in micro-fluidics: from gel emulsions to soft functional devices

We outline a concept of self-assembled soft matter devices based on micro-fluidics, which use surfactant bilayer membranes as their main building blocks, arrested in geometric structures provided by top-down lithography. Membranes form spontaneously when suitable water-in-oil emulsions are forced into micro-fluidic channels at high dispersed-phase volume fractions. They turn out to be remarkably stable even when pumped through the micro-fluidic channel system. Their geometric arrangement is self-assembling, driven by interfacial energy and wetting forces. The ordered membrane arrays thus emerging can be loaded with amphiphilic functional molecules, ion channels, or just be used as they are, exploiting their peculiar physical properties. For wet electronic circuitry, the aqueous droplets then serve as the ‘solder points’. Furthermore, the membranes can serve as well-controlled coupling media between chemical processes taking place in adjacent droplets. This is shown for the well-known Belousov-Zhabotinski reaction. Suitable channel geometries can be used to (re-)arrange the droplets, and thereby their contents, in a controlled way by just moving the emulsion through the device. It thereby appears feasible to construct complex devices out of molecular-size components in a self-assembled, but well controlled manner.

Solubilization of Thermotropic Liquid Crystal Compounds in Aqueous Surfactant Solutions

We study the micellar solubilization of three thermotropic liquid crystal compounds by immersing single drops in aqueous solutions of the ionic surfactant tetradecyltrimethylammonium bromide. For both nematic and isotropic drops, we observe a linear decrease of the drop size with time as well as convective flows and self-propelled motions. The solubilization is accompanied by the appearance of small aqueous droplets within the nematic or isotropic drop. At low temperatures, nematic drops expell small nematic droplets into the aqueous environment. Smectic drops show the spontaneous formation of filament-like structures which resemble the myelin figures observed in lyotropic lamellar systems. In all cases, the liquid crystal drops become completely solubilized, provided the weight fraction of the liquid crystal in the system is not larger than a few percent. The solubilization of the liquid crystal drops is compared with earlier studies of the solubilization of alkanes in ionic surfactant solutions.

Lasing and waveguiding in smectic A liquid crystal optical fibers

We demonstrate a new class of soft matter optical fibers, which are self-assembled in a form of smectic-A liquid crystal microtubes grown in an aqueous surfactant dispersion of a smectic-A liquid crystal. The diameter of the fibers is highly uniform and the fibers are highly birefringent. They are characterized by a line topological defect in the core of the fiber with an optical axis pointing from the defect core towards the surface. We demonstrate guiding of light along the fiber and Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the fiber. The light guiding as well as the lasing threshold are significantly dependent on the polarization of the excitation beam. The observed threshold for WGM lasing is very low (≈ 75μJ/cm(2)) when the pump beam polarization is perpendicular to the direction of the laser dye alignment and is similar to the lasing threshold in nematic droplets. The smectic-A fibers are soft and flexible and can be manipulated with laser tweezers demonstrating a promising approach for realization of soft photonic circuits.

Intermittent chaotic chimeras for coupled rotators

Two symmetrically coupled populations of N oscillators with inertia m display chaotic solutions with broken symmetry similar to experimental observations with mechanical pendulums. In particular, we report evidence of intermittent chaotic chimeras, where one population is synchronized and the other jumps erratically between laminar and turbulent phases. These states have finite lifetimes diverging as a power law with N and m. Lyapunov analyses reveal chaotic properties in quantitative agreement with theoretical predictions for globally coupled dissipative systems.

A Versatile Class of Cell Surface Directional Motors Gives Rise to Gliding Motility and Sporulation in Myxococcus xanthus

The Myxococcus Agl-Nfs machinery, a type of bacterial transport system, is modular and is seen to also rotate a carbohydrate polymer directionally at the spore surface to assist spore coat assembly.

Tuning active emulsion dynamics via surfactants and topology

Abstract.We study water-in-oil emulsion droplets, running the Belousov-Zhabotinsky reaction, that form a new type of synthetic active matter unit. These droplets, stabilised by surfactants dispersed in the oil medium, are capable of internal chemical oscillations and self-propulsion. Here we present studies of networks of such self-propelled chemical oscillators and show that the resulting dynamics depend strongly on the topology of the active matter units and their connections. The chemical oscillations can couple via the exchange of promoter and inhibitor type of reaction intermediates across the droplets under precise conditions of surfactant bilayer formation between the droplets. The self-emerging synchronization dynamics are then characterized by the topology of the oscillator networks. Further, we show that the chemical oscillations inside the droplets cause oscillatory speed variations in the motion of individual droplets, extending our previous studies on such swimmers. Finally, we demonstrate that qualitatively new types of self-propelled motion can occur when simple droplet networks, for example two droplets connected by a bilayer, are set into motion. Altogether, these results lead to exciting possibilities in future studies of autonomous active matter.Graphical abstract

Myelin Structures Formed by Thermotropic Smectic Liquid Crystals

We report on transient structures, formed by thermotropic smectic-A liquid crystals, resembling the myelin figures of lyotropic lamellar liquid crystals. The thermotropic myelin structures form during the solubilization of a smectic-A droplet in an aqueous phase containing a cationic surfactant at concentrations above the critical micelle concentration. Similar to the lyotropic myelin figures, the thermotropic myelins appear in an optical microscope as flexible tubelike structures growing at the smectic/aqueous interface. Polarizing microscopy and confocal fluorescence microscopy show that the smectic layers are parallel to the tube surface and form a cylindrically bent arrangement around a central line defect in the tube. We study the growth behavior of this new type of myelins and discuss similarities to and differences from the classical lyotropic myelin figures.